The present invention relates generally to optical scanner assemblies and laser imaging systems incorporating such scanners. In particular, the present invention relates to an internal drum type optical scanner assembly having a unibody laser beam power attenuation mechanism, including a variable density attenuator, suitable for use in a medical imaging system.
Laser imaging systems are commonly used to produce photographic images from digital image data generated by magnetic resonance (MR), computed tomography (CT) or other types of scanners. Systems of this type typically include a continuous tone laser imager for exposing the image on photosensitive film, a film processor for developing the film, and an image management subsystem for coordinating the operation of the laser imager and the film processor.
The digital image data is a sequence of digital image values representative of the scanned image. Image processing electronics within the image management subsystem processes the image data values to generate a sequence of digital laser drive values (i.e., exposure values), which are input to a laser scanner. The laser scanner is responsive to the digital laser drive values for scanning across the photosensitive film in a raster pattern for exposing the image on the film.
The continuous-tone images used in the medical imaging field have very stringent image-quality requirements. A laser imager printing onto transparency film exposes an image in a raster format, the line spacing of which must be controlled to better than one micrometer. In addition, the image must be uniformly exposed such that the observer cannot notice any artifacts. In the case of medical imaging, the observers are professional image analysts (e.g., radiologists).
Optical scanning assemblies are used to provide uniform exposure of the image on photosensitive film. The optical scanning assemblies combine a laser system with unique optical configurations (i.e., lenses and mirrors), for uniform exposure of the image onto the film. Past optical scanning assemblies used for achieving the level of performance required by the medical imaging industry utilize costly components incorporated into complex optical scanning systems. Such systems often combine complex, multi-sided mirrors and lens configurations for directing the laser beam onto a moving or stationary photosensitive film.
Known laser imagers used for medical imaging include a polygonal scanner or a galvanometer scanner. For example, a commonly used polygonal scanner configuration has a polygonal mirror repetitively exposing successive raster lines onto a sheet of moving photosensitive film. The optics must focus the laser beam over a flat image line and compensate for facet-to-facet angular errors in the polygon. These functions have usually been accomplished with combinations of costly precision ground anti-reflection coated glass lenses. The film is moved at a constant speed on rollers where the film is placed at the focus of the scanning laser beam. The film must be moved with a surface velocity constant to better than about 0.5%. Momentary disturbances or perturbations in the motion of the film, such as those caused by striking a film guide, position sensor or nip roller, can cause serious "glitches" in the exposed image, resulting in poor image quality. Avoidance of such perturbations requires that the film path during imaging be free of such obstructions. Such a film path often occupies a considerable amount of space in the laser imaging device.
Another known example of an optical scanner used for laser imaging includes a galvanometer optical scanner assembly having a dual-galvanometer configuration. The dual-galvanometer configuration includes one galvanometer mirror which repetitively sweeps the laser beam to form the raster lines, while a second, slower-moving galvanometer mirror deflects the raster lines down the page of the photographic film. The film, held motionless during exposure, is usually held in a curved platen to avoid the necessity of flattening the image field in both directions. While problems due to film motion are eliminated, since the film can be curved in only one direction at once, such a configuration requires the use of field-flattening optics for its operation. The use of galvanometers, on the other hand, offers relief from the problem of facet-to-facet errors found in polygon-based scanner systems. Galvanometers, like accurate polygon spinners, are precision instruments, and therefore, are very expensive to manufacture.
In light of the known drawbacks of present laser imaging devices, it is desirable to have an optical scanner which does not rely on expensive mirror and optical configurations to compensate for errors inherent in the scanner design. It is desirable to have an optical scanner for use in a laser imager which does not require an extraordinary amount of space nor which requires space considerations due to the introduction of glitches from the film path. Further, it is desirable to have an optical scanner for use with a laser imager which meets the image-quality requirements of the medical imaging industry.